Progression through the mammalian cell cycle is driven by orderly activation of cyclin-dependent kinases (CDKs). An active CDK is composed of a catalytic subunit and a regulatory subunit called cyclin. CDK activity is regulated through interactions with cyclins and CDK inhibitors (CKIs) and by post-translational modifications (e.g., phosphorylation).
Transition between cell cycle states is regulated at defined checkpoints by different cyclin subunits: G1 cyclins for the G1/S transition, S cyclins for progression through S phase, and G2 or mitotic cyclins for entry into mitosis. The commitment of a cell to enter the S phase occurs at a restriction point (R) late in G1, after which mitogenic growth factors are no longer required for cells to complete division.
Cyclins D and E are synthesized sequentially during G1 and are rate-limiting for S-phase entry, so they can be viewed as G1 cyclins. At least three mammalian genes encode D-type cyclins (D1, D2 and D3). D-type cyclins are progressively induced as part of the delayed early response to mitogenic stimulation, and they are expressed in a cell lineage-specific fashion. Assembly of D-type cyclins with CDK4 and CDK6 is regulated post-translationally by mitogens. Once assembled, cyclin D-bound CDKs must be phosphorylated by a CDK-activating kinase (CAK) to acquire catalytic activity.
Cyclin D genes are on a different branch of the evolutionary tree from A-, B-, or E-type cyclin, and D-type cyclins have some different properties from other cyclins. They are short-lived proteins (t.sub.1/2 &lt;25 min). Withdrawal of growth factors during G1 prevents steady accumulation of cyclin D, correlating with the failure of growth factor-deprived cells to progress past the R point. Thus, expression of cyclin D is regulated by extracellular signals, unlike the periodic expression of cyclins A, B and E.
Overexpression of human cyclins D1 and E in rodent or human fibroblasts shortens G1, decreases cell size, and reduces the serum requirement for the G1-to-S transition (Resnitzky et al, MCB ,4 1669-79 (1994); Quelle etal., Genes & Development 7, 1559-71 (1993); Ohtsubo & Roberts, Science 259, 1908-12 (1992)). These results suggest that D cyclins might override a function physiologically regulated by cyclin E or vice-versa. However, overexpression of cyclin D or E does not lead to fibroblast transformation--cells remain serum-dependent, contact-inhibited, and unable to form colonies in semisolid medium. Overexpression of cyclin D1 has also been found to enhance endogenous gene amplification, suggesting that it plays a role in genomic instability during tumor development. Zhou et al., Cancer Res. 56: 36-9 (1996).